This invention relates to diving masks and camera windows. In particular, this invention relates to underwater diving masks and camera cases having the improved underwater viewing characteristics of unit magnification and perspective effects approaching normal unaided vision in air.
A well-known phenomenon that creates angular magnification and perspective distortion is caused by the medium transition from water to air through a plane-parallel layer of tempered glass or plastic (see Human Underwater Vision: Physiology and Physics by J. S. Kinney, pg. 83-106). Such transitions are common in diver face-masks or flat optical windows used to protect a camera and electronic parts of cameras from the water environment. This effect is also visible looking down into still water and can be seen to be a function of the change in medium rather than the material of the optical window.
This effect can be derived from Snell's law of refraction: EQU n.sub.water sin (.theta..sub.water)=n.sub.window sin (.theta..sub.window)=n.sub.air sin (.theta..sub.air)
where n.sub.water is the index of refraction of water and .theta..sub.water is the angle from the normal of the surfaces where the light ray changed media between water and the window. The other variables are defined similarly. The indices of refraction are usually slightly adjusted so that the index of refraction in air becomes by definition exactly unity.
The window material turns out to be unimportant and the net effect of Snell's law in the transition from water to air is that there is a paraxial angular magnification of about 1.34 and binocular triangulation is subverted. This makes objects look nearer than they would look if they were perceived through air (i.e., perspective distortion). The magnification problem is particularly acute when attempting to view objects off-center in the optical system. There is an angle at which objects can no longer be seen from the air side of the window. Theoretically it is over 48 degrees but practically it is less than 45 degrees. Reports of 50-degree or greater fields of view result from slight motions of the head. Light having incidence angles greater than the value of somewhere between 48 and 49 degrees is totally reflected at the interface.
While divers have used flat face-masks for many years with good success, there are situations where correction of the above problems would prove to be beneficial. Divers having to do work underwater requiring a great deal of hand-eye coordination, such as underwater welding or salvaging, could enhance the fine control of their hands with their vision behaving as it does in air. The invention to be described in detail below performs the desirable demagnification required to achieve this, and does so without adding net optical power.
The prior art has failed in its attempt to achieve this desirable result. The invention of U.S. Pat. No. 3,040,616 provided an underwater goggle that had a paraxial unit magnification about equal to unity. It had severe off-axis aberrations of astigmatism and field curvature, however, reaching a full degree or more of blurring at 38 degrees. As long as mask users held the object rigidly at the center of the field-of-view of their goggles, they probably perceived adequate performance, but the off-axis performance limited this otherwise excellent design to the central ten degrees or so.
More recently, U.S. Pat. Nos. 5,359,371 and 5,523,804 have disclosed a thin negative singlet design. However, the claims of optical unit-magnitude performance of this device are incorrect. No thin lens of the optical powers specified in the patents (when placed only a few inches before the eyes) results in unit magnification. Perhaps the design of the patents relies on subtle and complex psycho-visual cues to partially correct perspective errors, but the fundamental optical design does not do this correction. The design of the patents also adds a small amount of negative optical power to the system, which can be accommodated by young viewers, but for older people pushes the limit of close-focus outward even farther than presbyopia allows in air. Corrective lenses can be placed in contact with the flat inner surface, but doing so would destroy any benefits of the very slightly curved outer surface.
More common are the attempts to correct for the intense negative power of dome-style lenses. Inventors of such devices reason correctly that domes are not limited in field-of-view like flat lenses. However, a dome small enough to fit over each eye individually has much stronger optical power than the normal eye is capable of refocusing by accommodation; a lens must be added internally to add some positive power to the system. Thus, the user recovers an approximately afocal beam. Inventions exemplifying this principle are disclosed in U.S. Pat. Nos. 3,672,750 and 5,625,425. The inventions of these patents are designed for widening the field-of-view, however, and not unit magnification.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a lens system adaptable to face masks for divers and camera windows that assures improved viewing characteristics underwater, including unit magnification and good perspective rendering over a wide field of view.